Liquid extraction processes are widely used for recovering and concentrating valuable components, in particular REM.
Isolation of REM concentrate from nitrate solutions using organic extractants based on neutral organophosphorous compounds (preferably tributylphospate—TBP) and isolation of REM concentrate from chloride solutions using organic extractants based on organophosphorous acids (preferably di-(2-ethylhexyl)phosphoric acid—DEHPA) is disclosed in the art (e.g. Chemistry and technology of rare and trace elements (rus). Part 2. Ed. Bolshakov K. A., Moscow, High School, 1976, p. 360; Slavetsky A. I. etc. Extraction of REM by DEHPA and TBP. Radiochemistry (rus), 1989, No. 1, p. 25-31; Stryapkov A. V., Salnikova E. V. Influence of amine nature on REM extraction by a mixture of amines and alkylphosphorous extractants. Vestnik OGU (rus), 2004, No. 5, p. 121-124).
The researchers pay considerable attention to the transfer of certain compounds from aqueous solutions into organic (extraction processes) and substantially less attention to the reverse processes (re-extraction processes).
A method for extracting rare-earth elements from phosphogypsum by treatment with nitric acid and subsequent extraction of rare earth elements (REE) by phosphine oxide is described in Martynova I.N. et al. Research of distribution of REE in the course of extraction from acidic nitrate-phosphate solutions. Collected articles “Processing and physico-chemical properties of compounds of rare elements. Apatity, 1984, pp. 6-8 (Rus). The disadvantage of this method is the need for expensive trialkyl phosphine oxide and the impossibility of complete liquid-phase removal of REE from the organic phase. Furthermore, because of the high loss of trialkyl phosphine oxide with the aqueous phase, this method is uneconomical and requires additional facilities for trialkyl phosphine oxide utilization.
A method for extracting rare metals from mixtures thereof using countercurrent re-extraction with a mineral acid solution, in particular distilled water acidified to a concentration of 0.05 mol/L with HNO3 or acidified to a concentration of 1.0 mol/L with hydrochloric acid is described in RU patent 2049133. The disadvantage of this method is incomplete REM recovery and obtaining REM in form of a dilute solution of low concentration.
A method for processing phosphate rare earth concentrate comprising treatment of the concentrate by heating in the presence of an excess of oxalic acid to precipitate oxalates of REM is described in RU patent 2344079. However, this method is limited due to use of expensive oxalic acid.
Thus, REM re-extraction is usually carried out with concentrated acids, water (in particular, at an elevated temperature), carbonate or alkaline solutions. Liquid phase re-extraction with concentrated acids is suitable for processes with organic extracts having a high content of REM. If the REM concentration in the organic phase is low, as often happens at the extraction of the target component from dilute salt solutions having a high content of impurities, acid re-extraction allows solutions of low concentration up to 10 g/L of REM to be obtained.
Re-extraction with carbonate or alkaline solutions allows one to obtain a concentrated precipitate of REM, however it is associated with losses of acid passed into the organic solution during the REM extraction step, significant consumption of re-extractant, the appearance of large volumes of solutions of alkali metal salts or ammonium salts, with increased losses of the organic extractant due to its increased solubility in an aqueous phase. Water re-extraction possesses all these disadvantages, and re-extraction with oxalate solutions is limited due to the use of expensive oxalic acid.